Conventional railroad crossings that include flashing lights, audible alarms and moving arms that block a road crossing help ensure that automobiles do not cross a rail-road track while a train is approaching. However, such conventional railroad crossing apparatus is expensive to install and maintain, especially in the large numbers required along the length of railroad tracks across the continental United States.
Railroad warning systems that lack movable arms to physically obstruct the roadway are less expensive and are commonplace, especially in rural areas. However, in modern automobiles, the audible sounds may not be heard, especially if the driver is listening to a radio while driving. In such situations, a driver of an automobile may not hear the audible warning bells from the railroad crossing, which may result in a catastrophic locomotive-automobile collision. One solution is to replace the audible warning systems with a tone-modulated radio transmitter that will alert automobile drivers listening to their radio of an approaching train.
Simple tone-modulated warning transmitters which sweep the broadcast bands often sound like common man-made interference, thereby offering insufficient information to the listener to determine that there is an emergency and what action could or should be taken. Moreover, if the listener cannot determine that a train is approaching, such simple warning systems will only distract and annoy the listeners, causing them to be even less responsive to the approaching train. Also, the sweep or stepping of a single carrier through the broadcast band(s) favors simple alarm tone modulation as alarm voice messages would never be heard in their entirety if complete coverage of the broadcast band(s) by a single swept carrier is to be provided in reasonable time.
Full band transmissions which saturate the entire broadcast band(s) require significant amounts of carefully controlled radiated power to be effective at anyone frequency. Such high power requirements are not well suited to railroad crossing applications, where the power is typically supplied by battery and/or solar power arrays.